This invention relates to a remote control signal reproducing circuit for a remotely-controlled receiver which is used for remote control of an apparatus, such as a television receiver set, by means of an intermittent infrared beam.
As shown in block form in FIG. 1, a prior art remotely-controlled receiver comprises a light receiving unit 1 adapted to receive a modulated transmission signal of an infrared beam and convert it into an electrical signal, an amplifier unit 2 adapted to amplify a small amplitude of the modulated transmission signal of a converted electrical form, a filter circuit 3 adapted to eliminate noise, a detector circuit 4 which detects the modulated transmission signal and reproduces therefrom a remote control signal, a waveform shaping circuit 5, and a signal demodulator circuit 6 mainly comprised of a digital circuit. See for example, the article "Remote Control With MOS ICs For TV Sets" in the IEEE Transactions on Consumer Electronics Vol. CE-21, No. 4, Nov. 1975, pp311-319 and the ITT pamphlet, SAA 1050 and SAA 1051, "IC-kit for Infrared Remote Control", May, 1978.
To explain the remote control signal and the modulated transmission signal, an infrared beam is subjected to a pulse position modulation so that a plurality of remote control signals are generated for controlling a plurality of functions in a remotely-controlled device of the television receiver set, and the remote control signals and the modulated transmission signal have waveforms as exemplified in FIG. 2.
Specifically, FIG. 2 illustrates by a waveform (a) a remote control signal constituted by pulses of an identical pulse width which are arrayed at uneven periods. For example, a pulse of 1 ms pulse width at a pulse period of 2 ms is defined to represent "0" and a pulse of the same pulse width at a pulse period of 4 ms is defined to represent "1" so that a remote control signal is constituted by a plurality of pulses. Five pulses as shown by the waveform (a) in FIG. 2 set up a remote control signal representative of "0010". A modulated transmission signal as shown by a waveform (b) in FIG. 2 is obtained by producing a signal modulated at several tens of KHz only when the pulses for the remote control signal as shown by the waveform (a) in FIG. 2 are present. The modulated transmission signal is applied to a light emitting unit comprised of, for example, an infrared light emitting diode (not shown), and transmitted to the remotely-controlled receiver so as to act as a switching signal for the infrared beam.
When the above remote control signal and the modulated transmission signal are used, various kinds of remote control signal can be prepared by increasing the number of pulses, thereby making it possible to control a number of functions.
The light receiving unit 1 in FIG. 1 receives an optical signal of an infrared beam and produces a modulated transmission signal of an electrical waveform shown by (b) in FIG. 2. The modulated transmission signal is amplified at the amplifier unit 2 and its noise component is removed by the filter unit 3. Since the filter has a tuning frequency tuned to a modulation frequency of several tens of KHz, the filter unit 3 produces an output signal as shown by a waveform (c) in FIG. 2. The output signal is then detected by the detector circuit 4 and the waveform shaping circuit 5 reproduces from a detected signal a remote control signal as shown by the waveform (a) of FIG. 2, which in turn is applied to the demodulator circuit 6.
A prior art detector circuit 4 comprised of a diode and an integrating circuit can operate normally when the modulated transmission signal has a sufficiently large amplitude and is substantially devoid of noise. However, the filter unit 3 sometimes fails to eliminate large noise resulting from natural light and infrared rays emitted from an electric lamp which interfere with the modulated transmission signal that travels over a long distance. In such a case, the waveform shaping circuit 5 cannot effect correct reproduction of a transmitted remote control signal, resulting in erroneous operations. Conversely, correct operations can be assured only for a remote control over a decreased distance.
Further, the detector circuit 4 comprised of the integrating circuit is difficult to form as an integrated circuit, requiring an increased number of components and raising the costs.